The invention relates to a method and apparatus for determining expansive volumetric changes in curable materials. In particular, the invention relates to a method and apparatus for accurate on-site determination of expansive volumetric changes in relatively large aggregate size, hardened, shrinkage-compensated concrete. An example of shrinkage-compensated concrete is defined by America Concrete Institute Committee ACI 223. The test is operative without delay from the time of a concrete pour and employs a cylindrical container and a strain gauge for measuring hoop stress.
Portland cement used in concrete mixes results in a material that has about 0.04%-0.08% shrinkage over its lifetime. This results in surface cracking over time. Shrinkage-compensated concrete is a high performance material which, by means of expansive additives, compensates for normal shrinkage. This material is useful in applications where a high quality, crack free surface is required. In order to assure that the shrinkage-compensated concrete has a desired expansion characteristic, a test is needed to measure such expansion.
A currently available test for determining expansion of shrinkage-compensated concrete is set forth in ASTM designation C878. The test employs an elongated mold for casting a test specimen and a restraining cage consisting of a threaded steel rod and spaced-apart end plates secured to the rod. The restraining cage is placed in the mold and the material to be tested is compacted into the mold between the plates. After the material initially sets up or cures (six hours minimum), it is demolded and length measurements of the rod are taken at specific times over a period of seven days. At present, expansions in a range of 0.03 and 0.07 percent are considered to be suitable for reduction in ultimate shrinkage cracking in shrinkage-compensated concrete.
Although the C878 expansion test is the currently acceptable standard, it is difficult and costly to implement. For example, the test requires a skilled laboratory technician to perform it properly. The test procedure requires that the specimen is demolded not less than six hours after casting. In a field construction context, this means that the laboratory technician is operating on an overtime basis, or two shifts are required. Further, demolding fresh, weak, elongated concrete bars is a process requiring great delicacy and skill. It is not unusual for the molded sample to break during the demolding process. Also, there is a considerable delay between the time that the sample is cast and the first significant expansion measurement is taken, for example, an initial expansion measurement can only be taken after the sample is demolded. Consequently, bar expansions that occur after the concrete sets but before the first significant measurement may be made are not detected by the procedure.
The C878 test also places some limitation on the maximum size of coarse aggregate which may be employed in the samples. The mold in the ASTM test is 3".times.3".times.10" and the rod in the restraining cage is disposed centrally of the mold, limiting the size of the opening available for concrete entry and placement. This makes it difficult to fabricate samples with concrete containing large aggregate. In fact, the ASTM standard has a caution regarding the use of concrete containing coarse aggregate with a maximum size greater than about one inch.
Finally, measurement of length changes in the ASTM C878 test sample is time consuming and must be done in a laboratory by a skilled technician. Thus, there is a considerable expense involved.
It is therefore desirable to have available a method and apparatus for performing an on-site concrete expansion test. It is also desirable to have such a method and apparatus which requires less skill and time to perform as the currently available method. Further, it is desirable to have a method and apparatus for testing curable mixtures which provide instantly available information and which is not limited to the aggregate sizes as is the C878 test.